<!DOCTYPE html>
<html>
<head>
<style type="text/css">
#separator, #first-child {
 height: 20px;
 background-color: green;
}
#spacer {
 height: 10px;
}
</style>
</head>
<body>
<div id="separator"></div>
<div id="spacer"></div>
<div id="first-child"></div>
</body>
</html>
